1. Field of the invention
The present invention relates to novel supported catalysts for polymerizing ethylene and olefins having 3 or more carbon atoms, such as propylene, 1-butene, 1-pentene, 1-hexene and 1-octene. In particular, the present invention concerns a new method for manufacturing catalysts comprising at least one metallocene compound of a Group 4A, 5A or 6A transition metal on an inorganic oxidic support material, such as silica or alumina. The present invention also relates to catalysts prepared by said method, and to a process for polymerizing ethylene or other 1-olefins or mixtures thereof by using said catalysts.
2. Description of Related Art
Typical prior art catalysts for olefin polymerization are represented by the Ziegler-type catalysts, which contain mixtures of transition metal compounds and aluminum alkyl compounds on a suitable support, such as silica. The transition metal compounds of the Ziegler-catalysts are often comprised of titanium or vanadium compounds. Recently a new class of catalyst systems containing metallocene compounds of transition metals have been developed. In comparison to traditional Ziegler-catalysts, the metallocene catalysts have several advantages, in particular as regards their catalytic activity. Thus, especially catalysts comprising the reaction product of a cyclopentadienyl-transition metal compound and alumoxane have very high activities in polymerization of ethylene or other 1-olefins.
There are, however, some problems associated with the metallocene catalysts, as well. The metallocene catalysts were first developed for use as homogeneous systems, which have the disadvantage that they cannot be employed for polymerizations in the slurry or gas phase. Another disadvantage of the homogeneous systems resides in the fact that high activities can only be achieved by using very high amounts of alumoxane, thereby producing polymers with high aluminum content. More recently, supported systems have also been developed. These heterogeneous catalysts can be used both in slurry and gas phase polymerizations. For example, in EP 206.794 there are disclosed metallocene catalysts, which are prepared from the reaction product of a metallocene compound with alumoxane in the presence of an inorganic support material, such as silica, the main object being to decrease the amount of alumoxane used. However, the disclosure of said patent application shows that the use of lower alumoxane content in the catalysts leads to extremely low catalyst activities. Magnesium dichloride has also been used as support material for metallocene catalysts (see, e.g., EP 436 326).
All prior art methods for manufacturing metallocene catalysts comprise depositing the metallocene compound and the other components on the support from solutions or suspensions. This is disadvantageous because catalytically inactive compounds are introduced into the catalysts during the preparation process. These compounds have to be removed afterwards. The use of hydrocarbon solvents gives rise to environmental problems, and purification and recirculation of the solvents causes additional costs. Furthermore, it is difficult to prepare catalysts which have complicated structures.
Therefore, in view of the prior art, there exists a need for methods of preparing metallocene catalysts which do not resort to using solvents or other components which have to be removed from the finished catalyst systems. In addition, there exists a need for heterogeneous, high-activity metallocene catalysts which do not contain large amounts of activators, such as alumoxane, and which provide means for regulating the molecular weight and the molecular weight distribution of the polymers obtained.